Plywood has been processed into a hollow pipe through a process of bending two pieces of plywood into a semicircular shape and subsequent combining of them. Hollow pipes prepared by such method have been used for, for example, obtaining concrete columns by molding concrete in said hollow pipes.
FIG. 6 presents a conventional apparatus for bending plywood. It comprises a bed portion 100, a sheet supporting frame 102 which is mounted to be swivelled upwards and downwards with respect to the bed portion 100 around a pivot 101, a rubber sheet 103 which is supported in a spread state by the sheet supporting frame 102, mold 104 which has a semicircular cross section and is mounted on the bed portion 100 under the rubber sheet 103, a vacuum suction device 105 which is mounted on the bed portion 105 and makes said rubber sheet 103 contact tightly with the mold 104 by a negative pressure, and a heating hood 106 sliding in the horizontal direction and subsequently covering the sheet supporting frame 102.
The plywood (work) W to be bent is planar and consists of a plurality of elementary boards w1 laminated with uncured adhesive agent, as shown in FIG. 8. The work W is obliquely inserted between the sheet supporting frame 102 and the mold 104 while the sheet supporting frame 102 is swivelled up around the pivot 101. Then, the sheet supporting frame 102 is set down to the horizontal position and clamped to the bed portion 100. The rubber sheet 103 urges the work W into tight contact with the mold 104 when a negative pressure is applied. During this period, the elementary boards w1 are bent into semicircular shape slipping with respect to each other with the uncured adhesive agent. When the heating hood 106 presented in FIG. 6 is set and heats the work W for the defined period, the work W then maintains the bent shape due to the curing of the adhesive agent. Finally, the sheet supporting frame 102 is lifted away and the work W completed is taken out from the mold 104.
There are, however, several problems with such conventional forming apparatus. The first problem is that the work W is apt to be positioned at an angle with respect to the center line of the mold 104, as shown in FIG. 12, when the sheet supporting frame 102 is set down after the insertion of the work, as shown in FIG. 7. When the sheet supporting frame 102 is set down, the relaxed rubber sheet 103 contacts with the work W and dislocates it from the normal position. The work W is then formed incorrectly because it is dislocated.
The second problem is the difficulty of insertion of the work W between the rubber sheet 103 and the mold 104 because of the deflection of the relaxed rubber sheet 103, as shown in FIG. 7.
The third problem is that when the rubber sheet 103 is subjected to a vacuum suction, wrinkles S appear on the edge portions of the rubber sheet 103 extending beyond both ends of the mold 104 since the length of the rubber sheet 103 is longer than that of the mold 104 to cover the mold 104, as shown in FIG. 11. The wrinkles S appear repeatedly every time of forming so that cracks generate easily from the wrinkles, thereby the life time of the rubber sheet 103 becomes short.